Optical beam spectrometer with movable lens

ABSTRACT

A spectroscopic system is described that provides at least one of focus of an excitation beam onto a sample, automatic focus of an optical system of the spectroscopic system for collecting a spectroscopic signal, and/or averaging of excitation intensity over a surface area of the sample.

CROSS REFERENCE

This application claims the benefit of priority pursuant to 35 U.S.C.§119(e) of U.S. provisional application No. 60/987,001 filed 9 Nov. 2007entitled “Optical Beam Spectrometer,” which is hereby incorporatedherein by reference in its entirety.

BACKGROUND

Many spectroscopic measurement devices focus a light source on a sampleto achieve a useable signal. The spectroscopic measurement devices oftenrequire a very tightly focused beam so that a signal image of aparticular spot can pass though a small aperture. The small aperture isused to maintain a high level of spectroscopic resolution as it too isimaged onto a spectroscopic detector that includes a physical aperture,physically small detector, or pixel of a multichannel detector. Inpractice this has led to spectroscopic devices that have specializedprecise mechanical mechanisms for focusing of the beam onto the sampleor moving the sample to optimally place it with respect to the beam.

BRIEF DESCRIPTION

A spectroscopic system is described that provides at least one of focusof an excitation beam onto a sample, automatic focus of an opticalsystem of the spectroscopic system for collecting a spectroscopicsignal, and/or averaging of excitation intensity over a surface area ofthe sample.

In one embodiment, a spectroscopic system is provided comprising: alight source adapted to provide a beam of illumination; an opticalsystem comprising an optical element, such as a lens or a prism, adaptedto focus the beam of illumination on a sample and receive a spectroscopysignal from the sample; an electromechanical stage adapted to move theoptical element along a spectroscopic axis; and a controller adapted toreceive a plurality of spectroscopic measurements and control a focus ofthe optical element via the electro-mechanical stage based upon acomparison the plurality of spectroscopic measurements.

In another embodiment, a spectroscopic system is provided comprising: alight source adapted to provide a beam of illumination; an opticalsystem comprising an optical element, such as a lens or a prism, adaptedto focus the beam of illumination on a sample and receive a spectroscopysignal from the sample; an electromechanical stage adapted to move theoptical element of the optical system; and a controller adapted tocontrol the electro-mechanical stage to move the focused beam withrespect to a sample.

A method to use an electromechanical stage to focus an optical element,such as a lens or a prism, through a sum of the pixels covered by aspectral region is also described. In one embodiment anelectromechanical device uses an optical element, such as a lens or aprism, to move a focused beam across a surface of a sample to generate asampling of the surface. The sample of the surface, for example, maycomprise an average. In another embodiment, an electromechanical devicemoves a beam across a surface to achieve lower beam intensities at thesample during a signal measurement period. The latter can prevent heator photo-damage to the sample during the measurement period.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a change in beam size corresponding to a change in alens position of an example optical system.

FIG. 1B illustrates an example lens system of a spectroscopic system inwhich an illumination beam is imaged onto a sample through an aperture.

FIG. 2 shows a comparison of intensity of a fixed focused beam on asample with a focused beam moved with respect to the sample.

FIG. 3 illustrates an embodiment of a method to move a beam with respectto a sample.

FIG. 4 illustrates a positive effect of a moving aperture system inwhich a static beam causes burning or potentially other damage to thesample, while moving a beam with respect to a sample is able to scan thesample without causing any damage to the sample.

FIG. 5 illustrates an embodiment of an electromechanical optical stagefor controlling a movement of a lens of a spectroscopic optical system.

FIG. 6 illustrates an example optical signal summing (OSS) method thatcan be used to automatically determine an automatic focus location for aspectroscopic optical system.

FIG. 7 illustrates another embodiment of a method of providing anautomatic focus of a spectroscopic optical system that allows for theremoval of background fluorescence before summing the spectrum signalalong a dispersion axis.

FIG. 8 illustrates yet another embodiment of a method for providing anautomatic focus of a spectroscopic optical system using a spectralsignature from a Raman spectrum.

FIG. 9 illustrates one embodiment of an automatic focus system includinga free-space design.

FIG. 10 illustrates an embodiment of a system for moving a beam of aspectroscopic system with respect to a sample.

FIG. 11 illustrates an example of rastering a beam in a directionorthogonal to a spectroscopic beam axis.

FIG. 12 illustrates an example of a spectroscopic system for analyzingsurface heterogeneity.

FIG. 13 illustrates an example of a spectroscopic system.

DETAILED DESCRIPTION

Spectroscopic systems often have specialized precise mechanicalmechanisms for focusing of a beam onto a sample or moving the sample tooptimally place it with respect to the beam. These mechanisms, althoughthey can function well in certain instruments, can limit the applicationof the spectroscopic systems for hand-held use or for rapid analysiswithout specific mechanical adjustments depending upon their size oradjustment requirements.

FIGS. 1A and 1B illustrate aspects of example optical designs for asampling portion of a spectroscopic system. FIG. 1A, for example,illustrates a change in beam 12 size corresponding to a change in a lens14 position of an example optical system 10, such as an optical systemof a spectroscopic system. An optimal focus 16 of the optical systemoccurs at a focal point or focal area of the lens where the illuminationbeam is more tightly focused. The intensity of the beam at a sample 18is defined as the number of photons per second per area at the sample.As shown in FIG. 1A, the intensity of the beam at the optimal focus 16of the optical system 10 will produce the highest intensity level of thebeam 12 at the sample 18.

FIG. 1B illustrates an example lens system 20 of a spectroscopic systemin which an illumination beam 22 is imaged onto a sample 24 through anaperture 26. A signal reflected from the sample is also received by theoptical system 20 of the spectroscopic system and imaged onto theaperture 26 of the system. Within the spectroscopic optical system 20the reflected signal imaged on the aperture 26 is in turn imaged onto adetector of the spectroscopic system. The size of the aperture 26, inpart, is a factor in the resolution of the spectroscopic system. If thesample 24 is located at a distance from the optical system 20 where thebeam size is not sufficiently focused, for example, the image from thereflected signal will likewise be too broad to be completely imaged ontothe aperture 26 of the spectroscopic optical system 20.

Although a small beam size is desired at the sample 24 of aspectroscopic system, the small beam size may cause poor spectroscopicanalysis of a sample 24 (e.g., a heterogeneous sample) where only aportion of the sample 24 is illuminated by the beam. Pharmaceuticaltablets, for example, often comprise heterogeneous mixtures of activeingredients and inactive fillers. While a practical measurement would beaveraged over a surface of the tablet, the requirement for a tightlyfocused beam only measures a single location on the surface of thetablet. Thus, a single spectroscopic measurement may only detect aportion of the ingredients of the heterogeneous sample.

Another problem with focused spectroscopic measurement devices is thatthe focused light source may alter or even damage a sensitive sample. InRaman or fluorescence spectroscopy, for example, intense laser lightsources are used to illuminate the sample sufficiently to produce adetectable signal. Often, in order to acquire a signal with sufficientstrength to be detectable over detector noise, a long integration period(acquisition time) is required. This extended sampling time requireseven longer periods for which a sample is exposed to intense radiation.Sensitive samples, such as a sample comprising a surface with activenanoparticles may be photosensitive and damaged by the focused lightsource. Where specific active nanoparticles in a confined area of thesample are continuously illuminated over a time period to acquire asignal, the nanoparticles could become inactive. Similarly, a materialthat absorbs the laser radiation and heats up due to the samplingradiation could lose integrity.

In one embodiment, a focused beam may illuminate an increased area of asample by moving the focused beam with respect to the sample. The beam,for example, may be rastered or scanned across a surface of the sampleto increase the surface area of the sample being measured or to decreasethe intensity of the beam on a particular area of the sample over anequivalent acquisition period.

By moving the beam with respect to the sample, a beam may besufficiently focused to allow for a measured signal to be imaged on anaperture of a spectroscopic system and still sample a larger area of thesample or decrease the intensity of the beam at the surface of thesample. In one embodiment, for example, the beam intensity may beaveraged over an increased surface area of the sample. Since intensityis an amount of radiation per area, increasing the area of the sampleilluminated by the beam decreases the intensity at the sample over anequivalent acquisition period. Averaging the beam intensity over thesample also provides the ability to achieve high sensitivity whileincreasing the sampled area. This is illustrated in FIG. 2 showing agraph with intensity I on one axis and an X dimension on the other axis.In FIG. 2 it is shown that an equivalent intensity 30 can be isolated ina single spot 32 or can be distributed over a larger area 34 in the Xdimension to decrease the average intensity at the sample.

One embodiment of a method to move a beam with respect to a sample isshown in FIG. 3. In this embodiment, a larger area 36 on the sample maybe illuminated by the beam using a wedge-type prism 38 that when movedcauses the beam position 40 to move. By moving the wedge prism duringthe acquisition period the beam intensity can be effectively decreased.Rather than provide the full intensity of the beam on one spot of thesample, a beam that moves across a surface of the sample has anintensity over an acquisition period that is reduced by the beam spotsize divided by the area swept by the pattern produced by the wedgeprism 38.

Each of the embodiments of moving a beam with respect to a sample (e.g.,scanning or rastering the beam) takes advantage of an importantprinciple of optics—that light paths are reversible. The reversibilityof the path indicates that the signal radiation will come to the samepoint as the source. In other words, in an aligned system the wedge canmove without causing a misalignment of the signal with the spectroscopicaperture.

FIG. 4 illustrates a positive effect of a moving aperture system inwhich a static beam 42 causes burning or potentially other damage to thesample, while a moving beam 44 is able to scan the sample withoutcausing any damage to the sample.

Another method to achieve an increase in spectroscopic sampling areas isto position a lens onto an electromechanical optical stage. Such systemsare common on compact disk players to rapidly maintain optimal focus andalignment with the optical information on the disk. Such an exampleelectromechanical optical stage 50 is shown in FIG. 5. This stage 50 maybe used to rapidly raster a sampling beam across a sample and positionthe beam to produce the spectroscopic signal.

FIG. 6 illustrates an example optical signal summing (OSS) method thatcan be used to automatically determine an automatic focus location ofthe optics in a spectroscopic system. In one embodiment, for example, alens of the spectroscopic optical system can be moved incrementally asshown in FIG. 1B. At different locations, a spectrum 60 is taken of thesample to provide a spectrum signal. The spectrum signal is summed alonga dispersion axis of the signal (e.g., by horizontal binning of thepixels of the spectroscope detector) to provide a spectroscopic focussignal. The spectroscopic focus signal can then be used to automaticallydetermine a focus location. In one embodiment, the spectroscopic focussignal can be used to provide feedback to an electromechanical stage ofthe optical system, such as shown in FIG. 5. An automatic focus, forexample, may be defined by the position of the lens that provides thelargest spectroscopic focus signal representing a sum of the spectrumsignal along the dispersion axis. In this embodiment, the focus on thesample is determined to be where the largest sum of pixels is found, andthe autofocus electromechanical stage is controlled to focus the beam atthat distance. Where a high speed binning process is used in which theentire horizontal axis is automatically summed (“binned”) into a singlepixel of the detector, the method provides a high speed way toautomatically determine a focus and to provide a rapid control signalthat can be used to position the electromechanical stage.

FIG. 7 shows another embodiment of a method of providing an automaticfocus of a spectroscopic optical system that allows for the removal ofbackground fluorescence before summing a spectrum signal 70 along adispersion axis. This embodiment, for example, may be useful in sampleswhere the fluorescence signal maximizes at a different focus than thespectroscopic signal (e.g., a Raman signal). For example, fluorescencestems from exciting the sample at an electronic absorption. It ispossible that this signal will saturate or decrease at the point wherethe optical signal (e.g., Raman signal) is optimized.

FIG. 7 shows one example of a method to remove the backgroundfluorescence. In this embodiment, a simple derivative spectrum 72 can becalculated by shifting the spectrum 70 by a few pixels and subtractingthe shifted spectrum from the original unshifted spectrum. Thederivative spectrum 72 can then be converted into all positive numbersby taking an absolute value. This method may be of particular value, forexample, when examining samples contained in glass containers since itis often observed that glass can produce a significant fluorescence peakthat is unrelated to the sample inside the container. This alternativeembodiment would reduce the signal from the glass container and producean automatic focus onto the spectroscopic signal (e.g., Raman scatteringfrom the sample).

Yet another embodiment of a method for providing an automatic focus of aspectroscopic optical system is shown in FIG. 8. In this embodiment, aspectral signature that is always present in a Raman spectrum is used.For a Raman spectroscope this is the Rayleigh line at zero (0)wavenumbers 80. When the lens of the spectroscopic optical system ismoved to a new location, a Raman signal and a zero-order signal aretaken by the spectroscopic system. The zero-order signal, for example,may be determined via a grating of the optical system or a filter thatremoves the non-Raman portion of the signal received from the sample.The inset 82 shown in FIG. 8 shows how this signal will change with thedistance from an optimal focus. In this embodiment, the automatic focuscan account for the mixing of the Rayleigh signal with reflections oflaser light at the same spectral position that may not correlate withthe optimal focus.

FIG. 13 shows an example of an optical spectroscopic system 110 that maybe used to provide automatic focus of the system as described above withrespect to FIG. 8. In this embodiment, an automatic focus control systemoperates a lens 112 to move the lens 112 with respect to the sample 114.The control system receives feedback from a photodetector 116.

FIG. 9 illustrates one embodiment of an automatic focus system 90including a free-space design. In this embodiment, for example, a focuslens 92 mounted on an electromechanical stage may be used to move thelens toward and away from the sample 94. One such free-space collimatedbeam design is disclosed in U.S. patent application Ser. No. 10/859,372entitled “Raman Spectrometer” and filed on Jun. 1, 2004, which isincorporated by reference in its entirety. A free-space design providesan easily adaptable system that uses a focus lens 92 to focus theexcitation source onto the sample 94 and to collect the spectroscopicsignal (e.g., Raman scattered signal) from the sample 94. One feature ofthis particular embodiment is that it produces a collimated beam 96after the spectroscopic signal (e.g., the Raman scattered signal) iscollected. The collimated beam 96 aligns with an aperture 98 of thespectroscopic regardless of the focus lens location. The focus field,for example, may be moved in and out using electromagnetic fields orother control options. An automatic focus may be determined by any ofthe methods described above or other automatic focus systems.

The same electromechanical lens stage can be used to move the beam inother dimensions. This is illustrated in FIG. 10 where the focus lens100 is moved in an orthogonal direction to the spectroscopic axis of theoptical system (e.g., a raster field). As seen in FIG. 10, the movementof the lens in the direction orthogonal to the spectroscopic axis of theoptical system moves the beam up and down, while maintaining alignmentwith the spectrograph aperture. Free running the electromechanical stagemay cause the beam to move across the surface very rapidly at a ratefaster than the acquisition time of the spectrograph. This will averagethe signal across the surface across the surface and effectivelydecrease the power density.

FIG. 11 depicts an example of rastering of the beam in a directionorthogonal to a spectroscopic beam axis. The beam on the surface iseffectively enlarged by the ratio of the rectangular area divided by thespot size. This reduces the potential damage to the surfaces of a sampledue to high laser power density and will average over the surface (e.g.,where the sample includes a heterogeneous surface composition).

Rastering (or otherwise moving) the beam could also be used to increasethe beam time on a moving sample. For example, if a reader is being usedto follow a material as it is flowing, the beam can be translated at theflow rate to prolong its incidence on the sample. For example, readingmoving items (e.g. currency marked with a Raman-active (or otherspectroscopic-active) tag) may be accomplished by allowing the reader tofollow a spot on the moving items and the time that the laser is on thespot could be maximized by moving the beam at the same rate as themoving items.

Surface Examination

A further application of this device is a hand-held inspection system toexamine a surface along one dimension. As described above, a beam may bemoved along a surface of a sample to reduce power density of light atthe sample and/or to average a measurement over a surface of the sampleto include surface heterogeneities.

In another embodiment, a measurement may be taken to measure the profileof spectroscopic (e.g., Raman) changes over one dimension. In one ofmany possible applications, a measurement of stress or chemicalvariations at a junction between two materials may be taken. In thesemiconductor industry, for example, semiconductor junctions areuniversally used to produce desired changes in current flow. In additionto the desired electrical properties that a junction might produce,however, the junction may also produce undesired stress in thematerials. In the plastics industry, plastics are typically desired tobe in their lowest energy state such that warping, strain, and the likedo not occur at a later time. However, when two plastics are “welded”the welding process can induce strain in the two weld materials and thisstrain can be a site of failure in the product.

A conceptual drawing of the method is shown in FIG. 12. This figureshows how the same raster mechanisms described above can be used todetermine and record surface heterogeneity. The drawing shows twomaterials (1 and 2) joined at a junction (J 1&2). A measurement of theRaman spectrum along the axis produces a series of spectra along theaxis. It is well known in Raman spectroscopy that the spectra aredependent in stress or other environmental factors. A plot of a Ramanparameter of value (POV) versus distance rastered is shown in the insetin the top right. The POV could be a spectral correlation, shift in aRaman band, intensity of a Raman feature, or other value in the Ramanspectrum that can change at a junction.

This measurement could also be used to determine the degree ofheterogeneity in samples. A junction represents a single heterogeneity;however, many materials are composed of small crystalline structurecalled grains. The size and distribution of the grains controls thematerials properties. Or the sample may be composed of severalmaterials, the distribution of which controls the materials properties.The device to raster the laser across the surface could measure theseheterogeneities.

Although embodiments of this invention have been described above with acertain degree of particularity, those skilled in the art could makenumerous alterations to the disclosed embodiments without departing fromthe spirit or scope of this invention. All directional references (e.g.,upper, lower, upward, downward, left, right, leftward, rightward, top,bottom, above, below, vertical, horizontal, clockwise, andcounterclockwise) are only used for identification purposes to aid thereader's understanding of the present invention, and do not createlimitations, particularly as to the position, orientation, or use of theinvention. Joinder references (e.g., attached, coupled, connected, andthe like) are to be construed broadly and may include intermediatemembers between a connection of elements and relative movement betweenelements. As such, joinder references do not necessarily infer that twoelements are directly connected and in fixed relation to each other. Itis intended that all matter contained in the above description or shownin the accompanying drawings shall be interpreted as illustrative onlyand not limiting. Changes in detail or structure may be made withoutdeparting from the spirit of the invention as defined in the appendedclaims.

What is claimed is:
 1. A spectroscopic system comprising: a light sourceadapted to provide a beam of illumination; an optical system comprisingan optical element adapted to focus the beam of illumination on a sampleand receive a spectroscopy signal from the sample; an electro-mechanicalstage adapted to move the optical element along a spectroscopic axis;and a controller adapted to (i) receive a plurality of spectroscopicmeasurements taken with the optical element disposed in differentlocations along the spectroscopic axis and (ii) control the focus of thebeam of illumination via the electro-mechanical stage based upon acomparison of the plurality of spectroscopic measurements; and, whereinthe controller is adapted to sum each of the plurality of thespectroscopic signals along a dispersion axis and compare the sums.
 2. Aspectroscopic system according to claim 1 wherein the controller isadapted to sum each of the spectroscopic signals along a dispersion axisvia a horizontal bin of a detector.
 3. A spectroscopic system accordingto claim 1 wherein the optical element comprises a lens.
 4. Aspectroscopic system according to claim 1 wherein the spectroscopicsystem comprises a hand-held spectrometer.
 5. A spectroscopic systemcomprising: a light source adapted to provide a beam of illumination; anoptical system comprising an optical element adapted to focus the beamof illumination on a sample and receive a spectroscopy signal from thesample; an electro-mechanical stage adapted to move the optical elementalong a spectroscopic axis; and a controller adapted to (i) receive aplurality of spectroscopic measurements taken with the optical elementdisposed in different locations along the spectroscopic axis and (ii)control the focus of the beam of illumination via the electro-mechanicalstage based upon a comparison of the plurality of spectroscopicmeasurements; and, wherein the optical element comprises a prism.
 6. Aspectroscopic system according to claim 5 wherein the prism comprises awedge prism.
 7. A spectroscopic system comprising: a light sourceadapted to provide a beam of illumination; an optical system comprisingan optical element adapted to focus the beam of illumination on a sampleand receive a spectroscopy signal from the sample; an electro-mechanicalstage adapted to move the optical element along a spectroscopic axis;and a controller adapted to (i) receive a plurality of spectroscopicmeasurements taken with the optical element disposed in differentlocations along the spectroscopic axis and (ii) control the focus of thebeam of illumination via the electro-mechanical stage based upon acomparison of the plurality of spectroscopic measurements; and, whereinthe electro-mechanical stage is adapted to move the optical element toraster the beam across the sample.
 8. A spectroscopic system comprising:a light source adapted to provide a beam of illumination; an opticalsystem comprising an optical element adapted to focus the beam ofillumination on a sample and receive a spectroscopy signal from thesample; an electro-mechanical stage adapted to move the optical elementalong a spectroscopic axis; and a controller adapted to (i) receive aplurality of spectroscopic measurements taken with the optical elementdisposed in different locations along the spectroscopic axis and (ii)control the focus of the beam of illumination via the electro-mechanicalstage based upon a comparison of the plurality of spectroscopicmeasurements; and, wherein the electro-mechanical stage is adapted tomove the optical element to scan the beam across the sample.
 9. Aspectroscopic system comprising: a light source adapted to provide abeam of illumination; an optical system comprising an optical elementadapted to receive the beam of illumination along a spectroscopic axis,focus the beam of illumination on a sample and receive a spectroscopysignal from the sample; an electro-mechanical stage adapted to move theoptical element of the optical system; and a controller adapted tocontrol the electro-mechanical stage to alter a path of the beam ofillumination with respect to the spectroscopic axis by moving theoptical element with respect to the spectroscopic axis.
 10. Aspectroscopic system according to claim 9 wherein the optical elementcomprises a lens.
 11. A spectroscopic system according to claim 9wherein the optical element comprises a prism.
 12. A spectroscopicsystem according to claim 11 wherein the prism comprises a wedge prism.13. A spectroscopic system according to claim 9 wherein the controlleris adapted to control the electro-mechanical stage to scan the focusedbeam across a surface area of a sample.
 14. A spectroscopic systemaccording to claim 9 wherein the controller is adapted to control theelectro-mechanical stage to raster the focused beam across a surfacearea of a sample.
 15. A spectroscopic system according to claim 9wherein the electro-mechanical stage is adapted to move the opticalelement orthogonally to the illumination beam.
 16. A spectroscopicsystem according to claim 9 wherein the optical element comprises awedge prism and the electro-mechanical stage is adapted to move thewedge prism with respect to the beam of illumination to change the pathof the beam of illumination.
 17. A spectroscopic system according toclaim 9 wherein the controller is further adapted to average a pluralityof spectroscopic signals received in response to moving the illuminationbeam across a surface of a sample.